An acceleration switch in which an inertia ball (a metal ball or the like) is housed in a metal container is disclosed, for example, by Japanese Patent No. 2892559 (prior art document 1) or Japanese Patent No. 2887556 (prior art document 2).
In these acceleration switches, an inertia ball is rollably housed in a metal housing serving as one electrode. The inertia ball normally remains stationary on a central bottom of the housing. A plurality of movable contacts comprised of contact members are provided uniformly all around the inertia ball. The movable contacts serve as the other electrode. The movable contacts are elastic and are adapted to elastically contact the inertia ball. Furthermore, the movable contacts are disposed relatively more densely around the inertia ball in order that operating characteristics of the acceleration switch may be prevented from being directional. Accordingly, the inertia ball rolls thereby to contact any one or more of the movable contacts when the acceleration switch is subjected to acceleration at or above a predetermined value. In addition, when remaining stationary on the central bottom of the housing, the inertia ball does not contact the movable contacts, whereupon a section between both electrodes (between the housing and the contact members) is rendered nonconductive.
When the acceleration switch is subjected to acceleration, deceleration or oscillation and a horizontal acceleration exceeds a predetermined value, the inertia ball rolls on the housing bottom thereby to contact the movable contacts provided therearound. When the inertia ball contacts the movable contacts, both electrodes of the acceleration switch are electrically connected via the inertia ball as a conductor to each other. Alternatively, a distal end of the movable contact displaced is brought into direct contact with the housing, whereby both electrodes of the acceleration switch are electrically connected to each other.
The following will describe the case where the acceleration switch is used for detection of seismic vibration. Regarding acceleration of vibration assumed in earthquake detection, an inertia ball generally reciprocates in the housing. Accordingly, the inertia ball repeats contact with and separation from the movable contact, so that the acceleration switch intermittently delivers a conduction signal (a signal corresponding to a conductive state of the acceleration switch) according to a vibration frequency or vibration level. Based on a duration of the conduction signal (an ON duration) or number of times, a determining device determines whether an earthquake occurred is at or above a predetermined level.
On the other hand, disturbance due to shock is sometimes given to an apparatus installed with the acceleration switch when a person or substance strikes the apparatus. In this case, a vibration frequency given to the acceleration switch depends upon a resonance frequency of the apparatus but is normally higher than frequencies of seismic vibration obviously. Accordingly, a duration of conduction signal intermittently delivered from the acceleration switch during contact opening and closure is rendered sufficiently shorter than due to an earthquake. As a result, the determining device can discriminate disturbance from an earthquake. Furthermore, when a duration of conductive signal exceeds a predetermined time period, the determining device can also determine that the acceleration switch is in an abnormal condition in which an apparatus installed with the acceleration switch has fallen over or inclined.
However, when a person or a substance happens to strike against an apparatus (a gas meter, for example) installed with the acceleration switch, a relatively larger disturbance is directly given to the acceleration switch. In this case, the determining device sometimes erroneously determines that an abnormal condition such as an earthquake has occurred. For example, as in the acceleration switch disclosed in prior art document 1, an inertia ball subjected to an acceleration due to shock rolls along an inner circumferential surface of the housing when the housing is cylindrical in shape. The inertia ball is usually in contact with any one of the movable contacts such that the conduction signal is continuous without interrupt. Accordingly, although there is actually no problem, the determining device erroneously determines that the apparatus installed with the acceleration switch is in a fallen state. Furthermore, when rolling elliptically, the inertia ball repeats contact with and separation from the movable contact, whereupon the determining device sometimes erroneously determines that an earthquake has occurred.
In view of the foregoing, an acceleration switch with protrusions on the inner circumferential surface of the housing has been proposed as in an acceleration switch disclosed by prior art document 2. According to the acceleration switch, a course of the inertia ball is disturbed by the protrusions even if the inertia ball rolls along the inner circumferential surface of the housing. As a result, the inertia ball cannot stably roll around in the housing but repeats contact with and separation from the movable contact, whereupon the conduction signal is delivered intermittently.
The above-described conventional acceleration switches are each structured so that the determining device can distinctly discriminate a conduction signal due to an earthquake from a conduction signal due to disturbance such as shock. However, even the foregoing conventional acceleration switches deliver a conduction signal corresponding with the conduction signal due to an earthquake or the like in rare cases under specified conditions, so that the determining devices make an erroneous determination.
This phenomenon occurs immediately before convergence of rolling of the inertia ball after the inertia ball has started rolling in the housing by disturbance vibration. The phenomenon is considered to occur immediately before convergence of rolling of the inertia ball when the inertia ball transfers to a circular motion within such a range that the inertia ball does not strike against the inner circumferential surface of the housing having protrusions. More specifically, when rolling, the inertia ball normally contacts the protrusions such that a rolling direction is disturbed, whereby the inertia ball is released from contact with the movable contact. However, when the inertia ball is caused to roll in such a range as not to almost flex the movable contact immediately before rolling convergence, the inertia ball remains in contact with the movable contact adjacent thereto without contact with the protrusion.
In view of the foregoing, a constructions has been proposed for preventing continuous contact between the inertia ball and adjacent movable contacts by reducing the number of movable contacts thereby to increase an intercontact distance or by providing protrusions (colliding portions) between movable contacts. However, when the number of movable contacts elastically contacting the inertia ball is reduced, a force suppressing the movement of the inertia ball (braking force) is reduced such that it becomes difficult to terminate the rolling of the inertia ball. As a result, the inertia ball is in contact with a single movable contact for a longer time period, whereupon a duration of the conduction signal is infrequently increased to or above a predetermined time period immediately before termination of the rolling of the inertia ball.
Thus, earthquake determination conditions are met such that a determining device determines erroneously even when the duration and the number of occurrences of the conduction signal meet respective predetermined conditions as well as when the duration of the conduction signal delivered from the acceleration switch has been increased to or above a predetermined time period. Particularly in collective housing areas, gas meters and the like are in many cases installed along aisles through which persons come and go. Accordingly, the gas meters are subjected to disturbance such as shock in many cases.